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BDC associations allow you to create master / child type relationships with the Business Data List Web Part and the Business Data Related List Web Part.

From the MSDN article, the following points should be followed:

• The entity that defines the association method must be below all of the entities to which it relates within the XML metadata file.
• The association method must have input parameters that map to the identifiers of all the source entities.
• The return parameter of the association method must include the identifiers of the destination entity.

I would recommend watching the associated 10 minute video from the MSDN article as well.